A PHYSICAL MODEL FOR LONG-TERM TRENDS IN PBO BOREHOLE TENSOR STRAINMETER DATA
DAY-LEWIS, A.D., email@example.com; ZOBACK, M.D., firstname.lastname@example.org, Department of Geophysics, Stanford University, Stanford, CA 94305, USA
After installation, tensor borehole strainmeter data routinely show relatively large magnitude, long-term strain signals that are normally removed from the data in order to identify lower-magnitude strain signals associated with seismic events or aseismic creep. Utilizing data from the earliest installed plate boundary observatory (PBO) borehole strainmeters, we show that these long-term trends, lasting up to a year or more, result from the poroelastic response to the stress concentration that arises from drilling the well into stressed rock. The availability of a fourth component of radial strain in the PBO strainmeters (compared to only three components at equal angles in earlier borehole tensor strainmeters) allows us to find a unique solution for the azimuthal distribution of accumulated strain at any given time. Short term strains during the first few months reveal the effects of grout curing, while the longer-term strains we are studying reveal information about regional stress directions (which can be confirmed with independent stress indicators) and the hydromechanical properties of the rock into which the strainmeter is installed.Our results will provide an alternative to the established procedure of removing a mathematically defined trend from each of the four borehole strainmeter gages individually. Because the current technique lacks a physical basis, it not only removes important geological information, but also may lead to erroneous interpretation of the residual strain signal. More generally, our results will reveal insight into time-dependent borehole deformation that will have implications for wellbore engineering and scientific applications.